I don't think you appreciate how much power is required for a supercharger station. Put it this way - if you had a SolarCity panel array the size of a Model S, it would produce enough power in a good day to add about 35 miles of range to a single car. Multiply that by the amount most people are actually charging when they pull in, and the number of cars utilizing a charger. It's just not even close to practical unless you cover the entire roof of a nearby building, and then you would need a huge battery somewhere to store the collected power for charging at night. Supercharger stations connect to the grid for a good reason.
How will this allow them to add more Supercharger stations? This will eat up their free cash flow which will actually allow for less spending on things like Supercharger stations.